Explanation

We often use question tags when we expect the listener to agree with our statement. In this case, when the statement is negative, we use a positive question tag. Hence it would read; We stood up when the principal came in didn't we

We often use question tags when we expect the listener to agree with our statement. In this case, when the statement is negative, we use a positive question tag. Hence it would read; We stood up when the principal came in didn't we